There is nothing lacking in the crucifixion of Christ considered in itself for his sacrifice was lacking in nothing and contained no defect being an infinite incomparable offering to God the Father.
Therefore, when Paul teaches that he makes up for what is lack in the Cross of Christ, he speaks in some qualified sense. Christ’s Cross is absolutely sufficient in every manner, but it is presented to men not as dominating the will but as demanding submission. The suffering that Paul embraces is nothing other than submission to the Cross of Christ, making present in his flesh that perfect sacrifice through submission to Divine Providence.
Suffering not only brings the will into submission of the reason through ridding itself of disordered affections, but in a greater way makes present through and by the Cross of Christ his sufferings and the salvation that he has won for us.
The rule of the eschaton is to suffer like Christ and so imitate his perfect submission and love of the Father or die eternally.